Wood is composed of three major constituents: cellulose, hemicellulose and lignin. Cellulose and hemicellulose are already exploited industrially, in particular in the paper industry. This use generates each year several million metric tons of lignin-rich by-products which are used as fuels of low heat-generating capacity for supplying heat and energy to paper production processes. In parallel, a minimum amount of lignin is isolated by direct extraction from plants (F. G. Calvo-Flores and J. A. Dobado, ChemSusChem. 2010, 3, pages 1227-1235).
Lignin is the most abundant substance, in terms of source, of aromatic groups in nature and the biggest contributor to the soil of organic matter (S. Y. Lin, in Methods in Lignin Chemistry, Springer Series in Wood Science (Ed.: C. W. Dence), Springer, Berlin 1992). It essentially results from the radical polymerization of three monomers called monolignols: p-coumarilyl alcohol, coniferyl alcohol and sinapyl alcohol, which, after polymerization by dehydrogenation with peroxidase, give respectively the p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) residues, as illustrated in FIG. 1 (R. Vanholme, K. Morreel, J. R. W. Boerjan, Curr. Opin. Plant Biol. 2008, 11, pages 278-285).
The complexity and also the diversity of the structure of lignin is dependent to a large extent on its origin. Using plant taxonomy as a basis, it has been proposed that the lignin derived from gymnosperms (called softwood) has more G residues than that derived from angiosperms (called hardwood), which contain a mixture of G and S residues, and the lignin derived from herbaceous plants contains a mixture of the three aromatic residues H, G and S. A more rigorous classification technique has been to use, as a basis, a chemical approach in which the lignins are classified according to abundance of the G, H and S units in the polymer. Four major lignin groups have thus been identified: G-type, GS-type, HGS-type and HG-type (F. G. Calvo-Flores and J. A. Dobado, ChemSusChem. 2010, 3, pages 1227-1235).
Whatever the lignin type, this biopolymer is characterized by a great biochemical heterogeneity and consists of propylphenol units bonded to one another by means of various types of C—O and C—C bonds of aryl ether, aryl glycerol and β-aryl ether type. FIG. 2 shows the structure of lignin proposed by E. Adler, Wood Sci. Technol. 1977, 11, page 169.
Ether bonds represent approximately two thirds of the bonds. More specifically, bonds of β-O-4 and α-O-4 type, which are among the alkylaryl ethers, are the most abundant. Typically, the lignin of angiosperms (hardwood) contains 60% of bonds of β-O-4 type and 6-8% of α-O-4 type, and the lignin derived from gymnosperms (softwood) contains 46% of bonds of β-O-4 type and 6-8% of α-O-4 type. Although the proportion of these bonds varies considerably from one species to another, typical values taken from M. P. Pandey, C. S. Kim, Chem. Eng. Technol., 2011, 34, 29, have been listed in the table of FIG. 3.
The chemical structures of the most abundant types of bond present in lignin are represented in FIG. 4.
Lignin represents the greatest renewable reservoir of available aromatic compounds. Because of its high content of aromatic compounds, lignin has a high potential to operate as an alternative to non-renewable fossil resources for the production of aromatic chemical products with a high added value, that is to say products of which the conversion considerably increases their commercial value. By way of aromatic chemical products with a high added value, mention may be made, for example, of 4-hydroxy-3-methoxybenzaldehyde or vanillin, 4-propylbenzene-1,2-diol or 4-(3-hydroxypropyl)-1,2-benzenediol.
Thus, the exploitation of lignin involves its conversion into precious and useful aromatic products. At the current time, few products, in particular aromatic products, that are pure or that can be easily purified, are obtained directly from lignin giving the very heterogeneous structure of lignin comprising several types of residues and of different bonds, the great difficulty in performing a selective cleavage of the lignin bonds, and also the difficulty in purifying the final mixtures obtained. Indeed, at the current time, the processes for producing aromatic products from lignin generate a large number of products which have physical and chemical properties that are similar to one another. Thus, separating them, in particular into a pure product, is very difficult using conventional purification techniques (chromatography or distillation). The only and most significant aromatic product obtained from lignin is 4-hydroxy-3-methoxybenzaldehyde or vanillin.
M. B. Hocking, J. Chem. Educ., 1997, 74, pages 1055-1059, has described the synthesis of vanillin from lignin sulfonates (lignin resulting from the sulfite process). However, this process has some drawbacks, in particular the production of large amounts of waste effluents (160 kg of caustic liquids for each kg of vanillin produced). Given the increasing awareness of the public regarding environmental issues, the costs of treating the non-sustainable effluents have become high and, consequently, factories using this vanillin synthesis process have begun to close.
Since 1993, the Norwegian company Borreegaard has constituted the only producer of vanillin from lignosulfonate. The key step of this process, initially developed by Monsanto, is the oxidation reaction which is carried out using a copper-based catalyst, said catalyst being subsequently recycled. This step is illustrated in FIG. 5.
This process uses an ultrafiltration technique coupled to a reverse osmosis technique, which makes it possible to reduce the volume of the waste streams and also to increase the vanillin yield (U.S. Pat. No. 4,151,207). This process makes it possible to obtain, from 1000 kg of wood: 400 kg (40%) of specialty cellulose, 400 kg (40%) of lignin, 3 kg (0.3%) of vanillin, 20 kg (2%) of yeast, 50 kg (5%) of ethanol and 45 kg (4.5%) of CO2, with exploitation of the heat generated by the process as described by F. G. Calvo-Flores and J. A. Dobado, ChemSusChem., 2010, 3, pages 1227-1235. The vanillin yield by means of this process is very low with respect to vanillin (less than 1%). Moreover, this process allows the synthesis of vanillin alone given that it is based essentially on the presence of G residues. The aromatic molecules derived from the other residues have not been exploited by means of this process.
The exploitation of lignin involving its conversion into precious and useful aromatic products continues to create a great deal of interest. The major problem for chemists is to develop, from lignin, processes:                which can deal with the variable characteristics of the structure of lignin, and        which make it possible to produce aromatic chemical compounds conventionally obtained by petrochemical methods, with good purity or which can be easily purified, and which will be able to serve as basic compounds in the production of fuels, electronic components, plastic polymers, rubber, medicaments, vitamins, cosmetic products, fragrances, foodstuffs, synthetic threads and fibers, synthetic leathers, adhesives, pesticides, fertilizers, etc.        
There is thus a real need for a process which allows the preparation of an aromatic compound, of good purity (at least 90% by weight, relative to the total weight of the aromatic compounds obtained) or which can be easily purified, from lignin, and which overcomes the drawbacks of the prior art.
In particular, there is a real need for a process which allows the preparation of an aromatic compound from lignin, said process:                making it possible to prepare aromatic compounds, in particular monocyclic aromatic compounds, with good purity (at least 90% by weight, relative to the total weight of the aromatic compounds obtained) or which can be easily purified;        having a selectivity which can be adjusted according to the aromatic compounds that it is sought to prepare;        being able to adapt to the versatility and heterogeneity of lignin, which depend on the type of vegetation from which the lignin originates and also on the process for the extraction thereof;        having a high productivity which results in a high conversion of lignin into aromatic compounds of good purity or which can be easily purified;        being able to be carried out under mild and industrially advantageous operating conditions; and/or        being simple to carry out.        
The development of a convergent depolymerization process may thus significantly contribute to the exploitation of lignin through its conversion into useful aromatic products while at the same time meeting the abovementioned requirements. The principle of a convergent lignin depolymerization process is to result in the reduction of the lignin into small oligomers which differ in terms of their degree of oxygenation. The latter are reduced (deoxygenated) gradually so as to converge toward a single product which accumulates in the reaction medium. However, lignin depolymerization is difficult and represents a challenge, since its structure is highly functionalized and branched and its steric hindrance can limit the access of the catalyst to the active sites. Furthermore, the chemical heterogeneity of lignin, which is due to the presence of several G, H and S residues present in variable amounts depending on the plant source, and to the presence of various types of C—O and C—C bonds of aryl ether, aryl glycerol and β-aryl ether type, complicates the obtaining of pure chemical products during the conversion of lignin.
Given the difficulty in performing a direct depolymerization of lignin, scientists have synthesized chemically pure models, representative of the ether bonds present in the lignin, in order to study the reactivity thereof (J. Zakzeski, P. C. A. Bruijnincx, A. L. Jongerius and B. M. Weckhuysen, Chem Rev., 2010, 110, page 3552). The majority of the studies targeting lignin depolymerization have focused on these models and have not been able to be carried out on the complex structure of natural lignins. Various organometallic complexes have been used to catalyze the reactions carried out on the models of ether bonds in lignin, for instance:                ruthenium (a) J. M. Nichols, L. M. Bishop, R. G. Bergman, J. A. Ellman, J. Am. Chem. Soc. 2010, 132, pages 12554-12555; b) A. Wu, B. O. Patrick, E. Chung and B. R. James, Dalton Trans., 2012, 41, page 11093; c) T. vom Stein, T. Weigand, C. Merkens, Jurgen Klankermayer, W. Leitner, ChemCatChem, 2013, 5, pages 439-441),        vanadium (S. Son and F. D. Toste, Angew. Chem. Int. Ed. 2010, 49, pages 3791-3794), and        nickel (AG. Sergeev and J. F. Hartwig, Science, 2011, 332, page 439).        
Given the complex, heterogeneous and highly hindered polymeric structure of lignin which complicates its depolymerization, the depolymerization processes developed in the literature are generally carried out under drastic temperature and pressure conditions and use metals in high catalytic amounts. Furthermore, processes that it has been possible to extrapolate from the chemically pure models to the depolymerization of lignin are rare. In 2013, the first organocatalytic reduction of model compounds of lignin was described by Feghali and Cantat (E. Feghali, T. Cantat, Chem. Commun., 2014, 50, pages 862-865). The latter showed that B(C6F5)3 is an efficient and selective hydrosilylation catalyst for the reductive cleavage of alkylaryl ether bonds and, in particular, of the models of α-O-4 and β-O-4 units. Furthermore, the reduction was carried out under mild conditions (ambient temperature for from 2 to 16 hours), and could be carried out with an inexpensive and air-stable hydride source such as polymethylhydrosiloxane (PMHS) or tetramethyldisilazane (TMDS). Nevertheless, this process could not be extrapolated to the depolymerization of lignin.
At the current time, there is no process for reductive depolymerization of lignin for preparing aromatic compounds. The only processes which describe the depolymerization of lignin in homogeneous catalysis are those carried out described by Toste et al. and Ragauskas et al.                It has been possible to extrapolate the process of Toste et al., which was developed on models of C—O bonds of the β-O-4 unit of lignin (S. Son and F. D. Toste, Angew. Chem. Int. Ed. 2010, 49, pages 3791-3794), to the lignin extracted from Miscanthus giganteus or elephant grass (J. M. W. Chan, S. Bauer, H. Sorek, S. Sreekumar, K. Wang, F. D. Toste, ACS Catal., 2013, 3, pages 1369-1377). In this process, a vanadium catalyst was used by Toste et al. for the cleavage of C—O bonds of the β-O-4 unit of lignin and the formation of aryl enones. This redox conversion is carried out in ethyl acetate at 80° C. for 24 h. The catalyst load is 10% by weight. The results of the dioxasolv and acetosolv lignin depolymerization CPG and 2D NMR studies resembled the data obtained with the lignin models, thereby confirming the selectivity for β-O-4 bonds. Finally, the authors were able to identify and quantify, by GC/MS, volatile phenolic compounds (such as vanillin, vanillic acid, syringic acid and syringaldehyde) produced in the reaction. Nevertheless, no chemical product could be isolated pure from this process and partially characterized complex mixtures were obtained with low yields for each product. Indeed, in the case of Toste et al., the depolymerization is carried out on grasses which are already made up of a mixture of the three residues H, G and S, and the process carried out is a redox process which, instead of converging the products obtained toward a single product, keeps the same diversity of products obtained as that present in the starting lignin.        The groups of Toste, Ellman and Hartwig grouped together their results on lignin reduction and models for the homogeneous catalysis thereof in international application WO 2011/003029. The precursors used are vanadium derivatives, ruthenium derivatives and rhodium derivatives. Only the vanadium-based and ruthenium-based complexes were used for the redox depolymerization of lignin extracted from Miscanthus giganteus. Nevertheless, no pure chemical product could be isolated or identified by means of this process and partially characterized mixtures were obtained.        In 2009, Ragauskas et al. (M. Nagy, K. David, G. J. P. Britovsek and A. J. Ragauskas, Holzforschung, 2009, 63, page 513) succeeded in depolymerizing organosolv ethanol lignin (EOL) (soluble in ethanol) derived from pine under reducing conditions. In this study, conventional heterogeneous catalysts and also new homogeneous catalysts were used for the cleavage of diaryl ether and dialkyl ether bonds. When using the hydrogenolysis conditions: 5 MPa H2; 175° C.; 20 hours, the ruthenium catalyst effectively increases lignin solubility (solubility up to 96%) and contributes to the degradation thereof. A decrease of about 10% to 20% of the weight-average molar mass (Mw) was obtained (Mw=1900-2100 g/mol), which corresponds to a degree of polymerization (DP) of 10 to 11 monomer units (L. B. Davin, L. B., N. G. Lewis, Curr. Opin. Biotechnol., 2005, 16, pages 407-415). Furthermore, according to the authors, the hydrogenolysis of the diaryl ether and alkylaryl ether groups is accompanied by a reaction of simultaneous hydrogenation of the aromatic ring and it is thus not possible to isolate aromatic compounds. Finally, the identification and also the detailed formation of the reaction products and of the cleavage pathways were not elucidated given that the products obtained are oligomers of which the structure is very difficult to identify. Ragauskas et al. neither mention nor obtain molecules of low molar masses.        
As already indicated, because of its high content of aromatic compounds, lignin has a great potential for operating as an alternative to non-renewable fossil resources for the production of aromatic chemical products with a high added value. However, because of its amorphous structure, which is very diversified in terms of residues contained, which use polymeric based on strong ether bonds, which contains a large number of bonds of different types, its polymerization to selectively produce usable molecules represents a challenge (P. J. Deuss, K. Barta and J. G. de Vries, Catal. Sci. Technol., 2014, Accepted Manuscript; DOI: 10.1039/C3CY01058A). In addition, lignins are structurally very diversified and, depending on the plant source used, they can contain different proportions of the three base monomers, namely p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol.
There is thus a real need for a process which allows the preparation of an aromatic compound from lignin and which can implement a lignin depolymerization step overcoming the drawbacks of the prior art.